1. Field of the Invention
This invention relates to a robot or program-controlled manipulator serving to assemble parts to a workpiece or object to be worked by fitting, inserting and screwing the parts into the workpiece or worked object, and more particularly, to a robot control system.
2. Description of the Prior Art
Heretofore, industrial robots such as teaching playback robots, for example, have been used in part assembling apparatuses to position the part in the same point and repeat the same work every time. Thus, assembling the part to the workpiece or worked object since they are generally excellent in repetitive playback performance. Therefore, it is necessary to teach the robot the points at which the success rate of assembling work is the highest, that is, the best working points, as teaching points.
The worked object such as substrate to which the part is to be assembled is fixed in a certain position by a positioning mechanism. However, due to limitations in the accuracy of the worked object and the performance of the positioning mechanism, the position in which the worked object is fixed is scattered or varied about a certain central value within a certain extent. For this reason, the central should be taught value to a tool or work executive section of the robot. Furthermore, since the central value varies in accordance with the lot of products (numbers of products) and change of the air temperature, it has been impossible for an operator to measure the central value for every work.
To cope with this, such a method has been proposed in Japanese Patent Unexamined Publication Nos. 60-196808 and 61-253508 that the position in which the worked object is fixed is measured beforehand using an exclusive distance sensor or visual sensor so as to correct automatically the teaching points of the work executive section.
In the conventional method, however, use of the distance sensor or visual sensor limited the worked object to such things that can be measured by the distance sensor or visual sensor. As a result, it was difficult to improve the success rate of the work with respect to the worked objects which cannot be measured by the distance sensor or visual sensor.
For example, in case of screwing up to plate members each having a coupling hole, only the coupling hole in the sensor-side (upper) plate member can be measured. Therefore, when the coupling holes in the upper and lower plate members were offset, the coupling hole in the lower plate member could not be measured, resulting in that the two plate members could not be screwed up even though the teaching point(s) of the work executive section was corrected according to the coupling hole in the upper plate member.
Further, according to the conventional method, since it is necessary to measure the position in which the worked object is fixed by means of the distance sensor or visual sensor for each work and since the measuring time is relatively long, the cycle time is prolonged disadvantageously.